It is well-known that impact-resistant vinyl aromatic copolymers are obtained by bulk polymerization or bulk-suspension polymerization, of solutions of olefinic rubbers, of the ethylene-propylene (EPM) or unconjugated ethylene-propylene-diene type, in a vinyl aromatic monomer alone or mixed with other copolymerizable ethylenically unsaturated monomers.
Styrene-acrylonitrile copolymers grafted on olefinic rubbers (AES) have excellent impact strength but their low thermoresistance makes them unsuitable for certain applications where temperatures of 120.degree.-160.degree. C. are used or for the preparation of products which come into contact with parts heated up to these temperatures.
To overcome this defect in AES copolymers, it is customary to mix these with other polymers having a good thermoresistance, such as a polyamide. This resin does, in fact, have good elongation, high energy absorption, especially in ball drop tests and excellent thermoresistance; however its great sensitivity to notching and high water absorption considerably limit the use of this resin. In addition, the polyamides are not very compatible with impact-resistant vinyl aromatic copolymers, consequently the interfaces between the domains of the components of the mixture represent extremely weak areas along which breakage of the material occurs.
To make polyamides compatible with the impact-resistant vinylaromatic polymers, Italian Patent Application 20921 A/90, filed on Dec. 17, 1990 by the present Applicant, proposed the use of a graft, impact-resistant, vinylaromatic copolymer containing in its chain units deriving from an ethylenically unsaturated comonomer having a carboxylic group or one of its derivatives.
The above Italian Patent Application relates to a thermoplastic composition including:
A) from 10 to 90% by weight of a graft, impact-resistant, vinylaromatic copolymer composed of a copolymer of a vinylaromatic monomer and of an acrylic ethylenically unsaturated monomer, and an olefinic elastomer, said copolymer formed by vinylaromatic monomer-acrylic ethylenically unsaturated monomer being at least partially grafted on the olefinic elastomer, and containing a grafted quantity of between 0.1 and 2% by weight of an ethylenically unsaturated monomer containing in the molecule a carboxylic group or one of its derivatives, such as anhydride, amide or ester; PA1 B) from 90 to 10% by-weight of a polyamide resin. PA1 A) from 10 to 90% by weight of a graft impact-resistant, vinyl aromatic copolymer composed of a copolymer of a vinylaromatic monomer and an acrylic ethylenically unsaturated monomer, and an olefinic elastomer, said copolymer based on vinylaromatic monomer-acrylic ethylenically unsaturated monomer being at least partially grafted on the olefinic elastomer, and containing a grafted quantity of between 0.1 and 2% by weight of an ethylenically unsaturated monomer containing in the molecule a carboxylic group or one of its derivatives such as anhydride, amide or ester; PA1 B) from 90 to 10% by weight of a polyamide resin; and PA1 C) from 1 to 10 parts by weight, with respect to the mixture of graft impact-resistant, vinylaromatic copolymer (A) and polyamide resin (B), of a polyolefin. PA1 100 parts by weight of an impact-resistant vinylaromatic copolymer composed of 90 to 30% by weight of a copolymer made up of a vinylaromatic monomer-acrylic ethylenically unsaturated monomer, at least partially grafted on 10-70% by weight of an olefinic elastomer, and PA1 from 0.1 to 2 parts by weight of an ethylenically unsaturated monomer containing in the molecule a carboxylic group or one of its derivatives such as anhydride, amide or ester; said monomer being grafted to said impact-resistant, vinylaromatic copolymer. PA1 Y represents hydrogen, a halogen or an alkyl radical having from 1 to 4 carbon atoms; and n is 0 or an integer between 1 and 5.
The compositions thus obtained, however, still do not have a combination of properties suitable for all the applications for which they are destined; in particular, the ultimate elongation is rather limited. This elongation can be improved by increasing the content of ethylenically unsaturated monomer containing a carboxylic group or one of its derivatives in the vinylaromatic copolymer; but this increase, however, prejudices other characteristics, such as the tensile modulus, especially for high contents of monomer containing a carboxylic group or one of its derivatives.